What is the reason that Encoding.UTF8.GetString and Encoding.UTF8.GetBytes are not inverse of each other?

Probably I am missing something, but I do not understand why Encoding.UTF8.GetString and Encoding.UTF8.GetBytes are not working as inverse transformation of each other?

In the following example the myOriginalBytes and asBytes are not equal, even their length is different. Could anyone explain what am I missing?

byte[] myOriginalBytes = GetRandomByteArray();
var asString = Encoding.UTF8.GetString(myOriginalBytes);
var asBytes = Encoding.UTF8.GetBytes(asString);